Disable unrolling for loops vectorised with non-constant VF

Disable unrolling for loops vectorised with non-constant VF

This is an alternative to https://gcc.gnu.org/ml/gcc-patches/2018-11/msg00694.html
As richi suggested, this disables unrolling of loops vectorised with variable-length SVE
in the vectoriser itself through the loop->unroll member.

It took me a few tries to get it right, as it needs to be set to '1' to disable unrolling,
the rationale for that mechanism is described in the comment in cfgloop.h.

	* tree-vect-loop.c (vect_transform_loop): Disable further unrolling
	of the loop if vf is non-constant.

	* gcc.target/aarch64/sve/unroll-1.c: New test.

From-SVN: r266281